Two-state electronic switches are widely used in modem electronics, from basic circuits to microprocessors and electronic memories. An electronic memory comprises a large number of ordered and electronically indexed switches, the state of each switch representing a binary “1” or “0” value or, in other words, a bit of information. Many different types of electronic memories are currently used in computers, in various types of intelligent electronic devices and controllers, and in many popular commercial products, including digital cameras and video recorders. While many applications require dynamic electronic memories that can be repeatedly written and read, many other applications require only static electronic memories that can be written only once, and then read repeatedly. Example write-once, read-many-times (“WORM”) memory devices include commonly used and optically accessed write-once compact discs, various well-known, read-only memories (“ROMs”), and various types of electronically accessed, microfuse-based crosspoint memories. WORM memories may be significantly less expensive than memories that can be repeatedly read and written, and may find great utility in applications requiring large, robust and resilient memory devices, including digital photography, where inexpensive WORM memories may be used for storing digital images in a fashion analogous to storing analogue optical images on photographic film.
FIG. 1 illustrates a recently disclosed type of organic-polymer-based memory element. The organic-polymer-based electronic-memory element comprises a p-type organic-polymer semiconductor layer 102 sandwiched between conductive signal lines 104–105. An additional n-type semiconductor layer 106 separates the p-type organic-polymer semiconductor layer from one of the signal lines, forming a pn-junction at the interface with the p-type organic-polymer semiconductor layer that acts as a diode to select for flow of current through the memory element in only one direction. In its initial post-fabrication state, the organic polymer layer is relatively highly conductive, conducting current with relatively low resistance between the two conductive signal lines, or conductive elements, when a voltage differential is applied to the conductive elements. This highly conductive state of the organic polymer constitutes a first stable state of the memory element that can serve to represent a binary bit “1” or “0,” depending which of two possible binary encoding conventions is employed.
A relatively high voltage pulse can be passed between the two conductive elements, resulting in a marked decrease in the current-carrying capacity of the organic polymer layer sandwiched between the two conductive elements. This change in conductivity of the organic polymer layer is generally irreversible, and constitutes a second stable state of the memory element that may be used to encode a binary bit “0” or “1,” again depending on which of two possible encoding conventions are employed, opposite from the binary bit encoded by the first stable state. A two-dimensional array of organic-polymer-based memory elements is obtained by forming a first layer of parallel, conductive signal lines, coating the first layer of parallel conductive signal lines with a layer of a first semiconductor, such as an n-type doped silicon layer, applying the organic polymer layer, and then fabricating a second layer of conductive signal lines on top of the organic polymer layer, the signal lines of the first layer oriented at an angle with respect to the signal lines of the second layer of generally between 30 and 120 degrees. Each overlap of a signal line in the first layer of signal lines with a signal line in the second layer of signal lines, along with the semiconductor and organic polymer layers between the signal lines of the first and second layer in the overlap region, constitutes a single memory element. The memory element is written with a high-voltage spike, to change the conductivity of the organic polymer layer from high to low, and is read at a low voltage sufficient to detect the difference in conductivity between the high conductivity and low conductivity states.
FIG. 2 shows the chemical structure of the PEDT/PSS polymer mixture, known by the trade name Baytron® P. The PEDT/PSS conductive polymer mixture is a mixture of a poly(3,4-ethylenedioxythiophene) polymer 202 and poly(styrene sulfonate) polymers 204. Baytron® P is prepared as an aqueous dispersion of a mixture of PEDT and PSS polymers. In general, the PEDT/PSS aqueous dispersion is spun onto a surface, to which it adheres upon drying to form an intrinsically conductive, transparent, and virtually colorless coating. PEDT/PSS has relatively high conductivity for an organic polymer, and can support current densities of greater than 200 amperes per cm2. PEDT/PSS has good photo stability and good thermal stability, and is relatively resistant to hydrolysis, and is therefore suitable for use as the p-type organic-polymer semiconductor layer 102 in the memory element illustrated in FIG. 1.
Unfortunately, repeated read access of memory elements of the type illustrated in FIG. 1 has revealed that, over time, even the low voltages employed for read access of the memory element result in degradation of the organic-polymer layer. When the memory element has, for example, a high conductivity state representing one of the two binary values, repeated read access may result in the memory element transitioning to a low conductivity state, and thus corrupting the information stored within the memory element. Designers and manufacturers of high-density electronic memories, and other memory-element-containing electronic devices, have therefore recognized the need for an organic-polymer-based memory element that is stable over repeated stored-information-access operations.